1. Field of the Invention
The present invention relates generally to radio frequency identification (RFID) tags.
2. Description of the Related Art
Many product-related and service-related industries entail the use and/or sale of large numbers of useful items. In such industries, it may be advantageous to have the ability to monitor the items that are located within a particular range. For example, within a particular store, it may be desirable to determine the presence of inventory items located on the shelf, and that are otherwise located in the store.
A device known as an RFID xe2x80x9ctagxe2x80x9d may be affixed to each item that is to be monitored. The presence of a tag, and therefore the presence of the item to which the tag is affixed, may be checked and monitored by devices known as xe2x80x9creaders.xe2x80x9d A reader may monitor the existence and location of the items having tags affixed thereto through one or more wired or wireless interrogations. Typically, each tag has a unique identification number that the reader uses to identify the particular tag and item.
Currently available tags and readers have many disadvantages. For instance, currently available tags are relatively expensive. Because large numbers of items may need to be monitored, many tags may be required to track the items. Hence, the cost of each individual tag needs to be minimized. Furthermore, currently available tags consume large amounts of power. Currently available tag power schemes, which include individually tag-included batteries, are inefficient and expensive. These inefficient power schemes also lead to reduced ranges over which readers may communicate with tags in a wireless fashion. Still further, currently available readers and tags use inefficient interrogation protocols. These inefficient protocols slow the rate at which a large number of tags may be interrogated.
Hence, what is needed is a tag that is inexpensive, small, and has reduced power requirements. Furthermore, what is needed are more efficient tag interrogation techniques, that operate across longer ranges, so that greater numbers of tags may be interrogated at faster rates.
The present invention is directed to an identification (ID) tag that is capable of operating in a radio frequency (RF) identification system. The RF identification (ID) system can be used to monitor and track items having the ID tag affixed to them. For instance, in a retail environment, a RF ID system may be desirable to determine the presence of inventory items that are located within the retail environment.
In one embodiment, the ID tag includes a substrate having an input capable of receiving a high frequency signal. For instance, the high frequency signal can be a radio frequency (RF) signal that is generated as part of a radio frequency (RF) ID system. A first charge pump is coupled to the input and disposed on said substrate. The first charge pump is configured to convert the high frequency signal to a substantially direct current (DC) voltage. A data recovery circuit is coupled to the input and is disposed on the substrate, where the data recovery circuit is capable of recovering data from the high frequency signal. A back scatter switch is coupled to the input and is disposed on the substrate, where the back scatter switch is capable of modifying an impedance of the input and is responsive to a control signal. A state machine is disposed on the substrate and is responsive to the data recovered by the data recovery circuit, where the state machine is capable of generating the control signal for the back scatter switch in response to the data. The DC voltage from the first charge pump is capable of providing a voltage supply for at least one of the data recovery circuit, the back scatter switch, and the state machine.
The data recovery circuit includes a second charge pump that is coupled to the input and disposed on the substrate, where the second charge pump is configured to retrieve data from the high frequency signal. A peak detector circuit is coupled to the output of the second charge pump, where the peak detector circuit is configured to generate a reference signal from the data signal. A comparator compares the reference signal to the data, to detect data transitions in the data. The first charge pump and the second charge pump are capable of simultaneously operating on the high frequency signal. In other words, the first charge pump can generate the supply voltage for the ID tag from the high frequency signal, while the second charge pump simultaneously retrieves the data from the high frequency signal.
The first charge pump includes a means for limiting an amplitude of the DC voltage. For instance, the first charge pump can include a metal oxide semiconductor field effect transistor (MOSFET) device having a threshold voltage based on a threshold amplitude of the DC voltage. The efficiency of the first charge pump is reduced when the amplitude of the DC voltage exceeds the threshold voltage causing the MOSFET device to conduct and shunt charge to ground. In one embodiment, the efficiency of the first charge pump is reduced by mismatching the input that receives the high frequency signal.